Because you already have one on-hand and that's the what the instructions for the pre-compiled ESC firmware you're trying to flash suggests?
FYI it works with an Uno, too.
I mean, there's a role for dedicated tools, and there's also a role for versatile solutions that use things most of us already own.
It so happens I bought a toolstick once upon a time, but for the past decade all I've been able to find is the relatively useless target half of it, the useful USB half hasn't been since in 3 moves But an Arduino? That's in my hand pretty darn quick.